There is known a method using a Phase Locked Loop (PLL) circuit in a local oscillator of a quadrature modulator-demodulator (modem) in a radio communication apparatus. Generally, in a local oscillator of a quadrature modem, a phase shifter is used so that an output signal of the PLL circuit and a signal with a phase difference of 90 degrees from the output signal are generated.
When local oscillation signals with a phase difference of 90 degrees from each other are generated, it is necessary to use a phase shifter as described above. There is a problem that a phase error in the local oscillator increases when accuracy of the phase shifter is worse.
Therefore, a PLL circuit using a Quadrature Voltage-Controlled Oscillator (QVCO) for outputting four-phase oscillation signals (differential signals with a phase difference of 90 degrees from one another) from one voltage-controlled oscillator by cross-coupling a pair of voltage-controlled oscillating circuits has been proposed. An example of such PLL circuit is disclosed in the following Related-Art Document 1 listed below.
Related-Art Document 1: IEEE Biomedical Circuits and Systems Conference, 2007, pp. 63-66
The PLL circuit disclosed in the Related-Art Document 1 uses a QVCO as an oscillator so that four-phase oscillation signals can be output without use of any phase shifter.
In the PLL circuit described in Related-Art Document 1, one of the four-phase oscillation signals output from the QVCO is however frequency-divided so as to be fed back to a phase frequency detector. When one (which is assumed to be a Q− oscillation signal here) of the four-phase output signals is input to a frequency divider, the load condition of the output terminal connected to the frequency divider, that is, at the output terminal A at which the Q− oscillation signal is output becomes different from the load conditions of the other output terminals B to D which are not connected to the frequency divider.
Accordingly, the phase difference and amplitude difference of the oscillation signal output at the output terminal A from the oscillation signals output at the output terminals B to D increase. That is, because the difference between the signal fed back and the real output signal increases, there arises a problem that the phase difference between the oscillation signals output from the PLL circuit is shifted from a required phase difference.